A Light Bridge to Nearby Stars

byPaul GilsteronAugust 23, 2013

by Charles Quarra

Charles Quarra tells me that like many kids who grow up dreaming about the stars, he realized early on that a career in physics would make sense. A book-filled childhood helped fix this focus, especially Carl Sagan’s The Cosmic Connection and Peter Nicholls’ The Science in Science Fiction. Charles went on to get a Physics B.Sc degree from Universidad Simón Bolivar in Caracas in 2006, but adds “pursuing it as a career turned out to be less satisfying as time went on. I’ve also been programming most of my life, so a transition to a software development career felt almost too natural for me, which I’ve been doing since moving to Panamá in early 2008.” His interest in space propulsion and exploration, though, remains strong, as evidenced by the paper he presented at Starship Congress in Dallas, which looks at building a chain of stable equidistant laser relays to provide beamed power for interstellar spacecraft. In the post below, Charles summarizes what he has in mind.

For decades since the beginning of the space age, mankind’s technological capabilities have grown exponentially in almost all but a few fronts of knowledge. One of the areas where evolution has been particularly frustrating is the result of the abysmal separation between our increasing capability to observe and find new potential places to visit, while our collective will to explore deeper and longer has stayed almost the same, if not languished at times. We seem to be the kids in front of the candy store of the universe, condemned to remain there for what seems like an eternity, while the object of our curiosity stays perpetually out of reach.

The ‘starway’ concept I present here is a natural evolution of the work of both Robert Forward and Geoffrey Landis in extending the reach of beamed power into deep interstellar space, by taming the beam divergence that is ever present in all laser wavefronts. Beamed power gives us the possibility of leaving the source of energy at home, avoiding the exponential blowout of energy requirements imposed by the Tsiolkovsky rocket equation. But beamed propulsion is far from devoid of issues: The pointing accuracy, the huge laser sources and sails tens or hundreds of kilometers wide demand engineering capabilities that are still far from our current horizon.

Handing Off Power to Starships

Conceptually, the starway tries to push the idea of multiple lenses for beam refocusing, analyzed by Landis in the 1990s, in the direction of making them reusable: Can we take a string of lenses, deploy them between two stars and keep them operational for long periods?

If so, the potential gains would be enormous: One of the problems that makes interstellar flight so hard is not only the extraordinary distances but also the almost entire absence of useful power required for propulsion. So a string of stabilized lenses in deep space would be able to deliver the required high energy density.

The conceptual step of stabilizing the lenses involves upgrading the elements to more than simple optical elements. We need slightly more complex relay devices capable of tasks that go beyond the simple optical refocusing of the beam: Reflection, beam splitting, heat rejection and telemetric coordination, to name a few other engineering requirements that the optical relays need to be able to do (sometimes simultaneously) in order to guarantee the continuity of power delivery along the structure, and toward nearby sails in transit. As it turns out, the principal parameter that determines the available power for sail propulsion on the light bridge structure is the optical efficiency of the relays: As the optical efficiency of the relays stays below the critical value (which for a starway made of thousands of nodes will imply losses per node around 100 ppm) the efficiency of the laser thrust utilization grows rapidly.

In order for sails to be able to take full advantage of the available power for maintaining constant acceleration without affecting their thermal envelopes significantly, material developments are needed: To handle the Doppler shifting of the beam relative to the sail rest frame, the sails either need to have reflectances with broad spectral peaks or they need to be made of tunable dielectric films that can electrically shift the peak. Dynamically adjusting the wavelength of the laser instead would not necessarily be a good strategy, since that would only translate the spectral efficiency problem from the sails to the optical relays, where efficiency is even more paramount in order to stabilize the starway.

Construction and Rationale

But does it make sense to deploy such structure? One of the vexing aspects in the starway architecture is the substantial requirement of deploying a laser system and solar collector array at the destination system. At first, the proposition seems ridiculous: If the capability is available to do it, why bother sending such substantial cargo instead of your actual intended payload?

The question can be answered only by specifying what we expect from interstellar flight. Do we want one-off missions, demanding huge investments with limited overlap and resource sharing? Or do we want to build a long-term infrastructure that, once deployed, consistently reduces the complexity and cost of subsequent interstellar flights? Interestingly, this is not the first time we had to deal with this same exact dilemma: Roman engineers perfected the technology to build large bridges over rivers that divided Roman territories and provinces. This technology was key to keeping the Roman Empire connected across most of Europe for several hundred years. Building bridges was undoubtedly a daunting task relatively compared to the effort required to cross the river in floating boats, but once they were deployed, the long-term benefits in reduced travel time and risk vastly exceeded any potential building costs.

Radial velocities will mean that certain stars are better suited for a starway than others. For instance, Gliese 581, at 20 light years, moves at a relatively slight pace of 10 kilometers per second. If we had 20,000 nodes (each at a 70 AU separation), we would have to add a new optical node in the starway every 30 years or so. For the same reason, Barnard’s Star (which is much closer, but has a velocity a lot higher than galactic escape velocity) would not be a good candidate for a starway, demanding several new nodes a year being pushed into the line.

As promising as the concept sounds, there are a number of issues that need to be solved before we can start building them. First, we need to figure out exactly what it would take to deploy such a structure. A starting point for deployment analysis is the multiple-lensed beamed sail mission proposed by Geoffrey Landis, and the fact that this analysis exists goes a long way to proving that deployment is far from being impossible. Depending on the masses of the hundreds or thousands of relays, deployment might take somewhere between several decades to a few hundred years. But the grunt work of relay design is still ahead, and graphene has an enormous potential for reducing the relay mass, at least for the thermal management sections.

An Interstellar Communications Network

But assuming these technical problems can be addressed, an advanced civilization could spread by deploying starways between neighbouring stars. A interstellar network of this kind would not only enable interstellar transport, but would also make high-bandwidth communications much more feasible. Might such a communications network be detectable?

On the problem of SETI by detecting the signature of remote starways in our galactic backyard, I’ve been fortunate enough to have had interesting exchanges with Adam Crowl, an interstellar expert who requires no further introduction to the Centauri Dreams readership, on the possibilities of detecting starways from remote distances. Unfortunately, typical relay nodes are expected to be far too small (a few kilometers at most, if relying on lasers on optically visible wavelengths) to be detected with our current observational resolution. Starways connected to our solar system would require a array laser and probably some extensive solar energy capturing infrastructure that we should probably have already detected if it was present at all.

But beyond a starway’s transportation capabilities, the architecture can easily be extended to relay information in bi-directional interstellar channels. This could provide a partial but nonetheless interesting explanation why purposeful ETI interstellar communications in radio bands seem to be largely absent: Communications are being exchanged by the equivalent of private end-to-end starway networks, largely out of the prying eyes of naive observers like us

Interestingly, the refocusing element design of a starway will profit from certain key developments in adaptive optics and optical mirror mass reduction that are highly sought by the astronomical community. In this regard, the unscheduled talk of Joseph Ritter from the University of Hawaii at Maui at Starship Congress was particularly enlightening, showing interesting possibilities for extending optical resolutions from the ground to the nano-radian range. Quoting my friend Miles Gilster, wouldn’t it be extraordinarily poetic if the technologies required to make the stars seem closer to us are the same technologies that will allow us to be closer to them?

Comments on this entry are closed.

GregAugust 23, 2013, 16:03

With talk about lasers, wasn’t there talk several years ago about using Mars’s atmosphere as the lasing source for a giant CO2 laser? I can’t seem to find it anywhere but I thought they mentioned approximately a terrawatt of laser power.

The best way to test this concept is to build something similar inside the solar system. It will give us practical experience and be useful in its own right. Imagine highspeed travel back and forth to Mars, various asteroids, and Titan. Of course, there is a special problem in-system, related to the constant motion of the destination objects but that can be addressed by having a fixed network of laser relays and choosing which branch to take at any given time.

My hope is that eventually you can wire up the whole solar system with laser power from interconnected beam relays very much like this. But the design complicates somewhat by all the gravitational influences, ray occlusion orbits, etc. At the moment is crucial to get some functional designs for the beam relay and estimate minimum masses. If you want to make the network truly propellantless, you need very light relays, mostly graphene skeletons with very-thin deformable mirrors. Otherwise you end up consuming too much power to adjust the orbits in deep-gravitational wells.

On an interstellar setting, you are less constrained by gravitational pull, but you STILL want a lightweight relay so that it doesn’t take 2 centuries to deploy the grid

1) For me, at least, the subject of the highest interest is attaining interstellar travel in the first place. This “starway” requires you to go to the other end first, and with lots of equipement, too, so it is just less interesting at this point in time. To me.

2) The angular precision that would be needed to project a near perfect beam (with less than 0.1% of light lost to anything, including scatter and diffraction) over the distances involved (70 AU) can only be mindboggling, and I have to assume it to be unachievable unless a fairly detailed treatment demonstrates otherwise.

Perhaps it would help if this posting could be supplemented with an actual reference, perhaps to the Landis work that is mentioned in passing? Landis would certainly have understood the optical limitations, so I am a bit surprised he would provide the basis for this.

1) Agreed. In fact I highlighted the tradeoffs on the article. This is an engineering project for when mankind gets bored of interstellar missions that take 40 years and they realise that they can do missions that take 15 years or so instead by investing in big infrastructure first

2) Actually the pointing accuracy required is at least 2 orders of magnitude better than the one required for the Forward decceleration stage at 10 light years. Besides, the latency between the beam alignment and a reply from the sail saying ‘beam alignment OK’ is not 20 years, but 16 hours instead

Why couldn’t be starway be the 1st interstellar flight? Why do we have to first go to the other end to then build the starway?

Couldn’t we just roll out the starway in a continuous series of launches, each launch using the previous object to decelerate to its proper position in the chain with the whole chain growing forward at essentially our top interstellar speed?

I am thinking once one has the relay set up in place one will need station keeping GN&C and propulsion because the Galactic Tide will perturb the configuration, but that can be done.
Placement does depend on the location of the destination, but would depend on wither there is planet there worth exploring or colonizing.
This might be a ‘follow on’ set up.
The observability of other civilizations building such an instrument is always of interest to me.

@David Cummings, that is exactly how Landis launch works. And is the best plan to deploy the starway. Some adjustments have to be made, and the energy requirements have to be recalculated, since we are not talking of just the optical elements or the sails, but a system of mirrors, etc. This is why I mentioned Joseph Ritter work: he is working on very lightweight deformable mirrors, in his case because as an Astronomer, he wants very good angular resolution in images to see distinct exoplanetary discs. I think this advancement will prove key in a few years, when more work and analysis on the starway system is done (hopefully)

I think CatharSeamus misunderstood my second point: I was not concerned about pointing accuracy (although this is, of course, also a problem), but optical precision, i.e. the requirement that at least 99.9% of the light needs to be directed accurately enough to hit the next lens. There is an incredibly small tolerance for angular deviation, such as could be caused by incredibly small inaccuracies in the lens’ shape or optical properties. From what I saw, Landis does not discuss the optical perfection needed for the lenses. Perhaps I missed it?

The stairway also promises to provide a superb baseline for extremely high-resolution VLBI, when we place a radio telescope at each station. That’s what I call “infrastructure”. When you lay down a train track, useful stuff tends to accrete along its length.

Eniac, sorry, I post from the mobile or the desktop, and CatharSeamus is my facebook name (long story)

If we assume the Gaussian beam is good enough to be diffraction-limited, the divergence is the ratio of the wavelength and the gaussian width. We currently have dielectric mirrors that can reflect a Gaussian beam with losses as small as 10^-5 *in the far field*, so this is already accounting diffraction losses. The challenge is of course, scaling this to one kilometer wide mirrors, without adding a lot of structural mass.

It is a daunting problem, but the fun part is that we are *already* working hard to develop this technology: Lightweight deformable mirrors is a holy grail for the astronomical community, in order to make large, cheap space telescopes.

Charles – I do not think that advanced civilisations will necessarily use the Starway for communications, because of the question you were asked about Gravitational Lensing in the meeting, after your talk. It is rather a shame that we did not have a presentation from Claudio Maccone on this subject, because it bears so much fruit for interstellar exploration. Basically, the radio power required from one focus at the far side of the target star to the focus on the other side of our sun (also where we should place a telescope) is of order one billion to one trillion times less than required otherwise. I don’t think that nondivergent beaming can compete with that.

@Andrew, regarding 1st comment on gravitational lensing, I entirely agree with you – It is hard to compete with the amplification of weak signals provided in the gravitational focal region

FOCAL observation stations would benefit from starways for easy energy delivery and reposition: as we know, one of the problems with gravitational focal observatories is that steering the position of the node is really hard.

But if a FOCAL observatory would sit between the angle spanned by two starways, the starways could provide beamed power to the observatories for making orbital adjustments, or simply powering a big flywheel, which would serve the dual purpose of storing energy and keeping the observatory aligned in the desired direction.

regarding 2nd comment, thanks for the kind words Andrew. The beam relays require, beside the refocusing lenses/mirrors, need a way to take a fraction of the beam and reflect it in a direction to provide thrust. A way to provide this is with a beam splitter coupled to a mirror. This same mechanism that will be needed to keep the relays in position, can be used to deliver the beam to the sails, that will fly in a designated region far enough to be safe from collisions from the relays, but still close enough to receive most of the beam. I would say that 10.000 km is more than enough for this purpose

Sail ships might be the spacecraft that first take human technology to distant stars.

Giant sails propelled by the sun’s or a laser’s energy could be the most viable option for interstellar spaceflight in the not-too-distant future, James Benford, a physicist associated with Icarus Interstellar, a non-profit group devoted to finding a way to travel to another star system, said during a panel at the Starship Congress conference in August.

Large and lightweight sails could allow unmanned probes to travel interstellar distances within a somewhat reasonable time frame, Benford said.

Theorists have discovered a new optical force that is analogous to the thrust that keeps aircraft aloft and causes tennis balls to swerve

If you’ve never heard of Bernoulli forces, you’ll certainly have experienced them. These are the forces that keep aircraft aloft, that draw fuel into your car’s carburetor and cause spinning tennis balls to swerve.

It is named after the 18th century Swiss scientist Daniel Bernoulli who discovered that a fluid flowing at high speed has a lower pressure than one flowing at lower speed. When the pressure difference occurs on opposite sides of the same object such as a wing, it experiences a force that pushes the wing from the high pressure region towards the low pressure one.

That raises an interesting question. Can objects sitting in an unconventional fluid-like flow, like a ligt beam, also experience Bernoulli forces? Today, Ramis Movassagh at Northeastern University in Boston and Steven Johnson at the Massachusetts Institute of Technology in Cambridge, say they can and explore the conditions in which this could happen.

These guys begin their analysis by imagining a rotating dielectric cylinder, like a glass rod, bathed in a stream of photons. In this analogy, the stream of photons is the fluid and the glass rod is the equivalent of a spinning tennis ball.

In a game of tennis, a spinning ball moving through the air swerves because the pressure on one side of it is greater than on the other. That’s because on one side of the spinning ball, its surface is moving into the airflow but away from it on the other side. That causes a difference in pressure, just as Bernoulli predicts.

Movassagh and Johnson ask whether a similar difference in pressure might arise for a spinning object in a beam of light. They conclude that it does but only if the object is made of a dielectric material, like glass or plastic.

In a dielectric, am external electromagnetic field can penetrate a short depth into the material. When the material is rotating, this interaction generates a force. Movassagh and Johnson calculate that this force is in the same direction as Bernoulli’s force when the material’s electric susceptibility is positive and in the opposite direction when the electric susceptibility is negative.

An interesting corollary is that the force is zero when the material is a conductor. That makes sense. “Because a perfect conductor allows no penetration of the electromagnetic ﬁelds, the ﬁelds cannot “notice” that it is rotating or be “dragged” by the moving matter,” say Movassagh and Johnson.

There is one caveat, however: this new optical Bernoulli force is tiny. However, they point out that it should be possible to magnify it by exploiting resonant effects. This could be done with multi-layered spheres that can trap light or by using materials in which the interaction with light is enhanced by surface plasmons, for example.

Nobody has ever seen the optical Bernoulli force but with this kind of magnification, it may be possible to see it the lab in the relatively near future.

The only question then will be what optical Bernoulli forces might be used for. Answers please in the comment section below.

In theory there’s no reason any region within the local galactic cluster, even isolated ones in deep space,could not be supplied with energy for a very reasonable fee or ha ha perhaps a great surplus of it if they try to be free riders. The general business plan is similar to my asteroid diversion one, where asteroids are not diverted into Earth in exchange for a very reasonable sum of money. And maybe the island of Hawaii.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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